Use the chart and go by size. My dd is thin, and I tend to size her down if she's on the line with sizes...for example, for her, 80's from last year still fit, especially shirts, but the dresses look a little short. 90's were way to big in the spring, but are ok now in dresses and shirts. She's thin, and no diapers so 90 pants are a little too generous, but in these last few weeks are working out. So it seems like other people experience this too...often you get a season and a half out of each item. I agree with HA, go by size not age. Their fabrics and excellent, durable, soft and wash and wear very well. With a slow growing child like mine, it works out great. If I had a very quick growing child and was not planning on handing down or reselling, it might not be as good a buy. I tend to be pretty good at getting out stains - and DD might be better then some children about making stains...so that's something else I would consider. I think HA's thicker fabric sometimes need more precise stain treatment. I have had good experiences with used HA items, too.
For a thin child some HA things are perfect, some are cut a bit too wide...like Land's End. Where Old Navy on the other hand tends to be cut too slim for my niece but generally perfect for DD.